1. Field of the Invention
This invention relates to a unit fuel injector for use in an internal combustion engine such as a diesel engine.
2. Prior Art
A unit fuel injector comprises a pump mechanism and an injection nozzle mechanism both of which are incorporated in an injector body, the unit fuel injector being mounted on an engine. The pump mechanism includes a cylinder bore formed in the body, and a plunger received in the cylinder bore for reciprocal movement therealong. A pump chamber is defined by the cylinder bore and the plunger. The volume of the pump chamber is reduced during an advance or pump stroke of the plunger, and is increased during a return or suction stroke of the plunger. The injection nozzle mechanism includes an injection port in communication with the pump chamber, and a valve disposed between the pump and the injection port. When the pressure of fuel within the pump chamber is increased to a high level during the pump stroke of the plunger, the valve is opened to inject the fuel from the injection port. The unit fuel injector is also provided with a fuel relief control mechanism by which the relief of the fuel pressure within the pump chamber is controlled during the pump stroke of the plunger so as to control the timing of starting the fuel injection and the timing of terminating the fuel injection. As one example of such fuel relief control mechanism, an electromagnetic valve mechanism has been proposed.
U.S. Pat. No. 4,674,461 discloses a unit fuel injector provided with such an electromagnetic valve mechanism. More specifically, there is provided a relief passage communicating at one end with a pump chamber, the other end of the relief passage being opened and closed by a needle-type valve member of an electromagnetic valve. In this conventional unit fuel injector, during the time when the fuel injection is being effected, with the relief passage closed, the valve member is subjected to a very high fuel pressure. Therefore, to cope with such a high fuel pressure, an associated solenoid is required to produce a sufficiently great force for holding the valve member in its closing position.
U.S. Pat. Nos. 4,392,612, 4,470,545, 4,485,969 and 4,527,737 also describe unit fuel injectors incorporating an electromagnetic valve mechanism. More specifically, such a fuel injector comprises a body which has a fuel supply and return chamber, a fuel supply passage and a drain passage, the fuel supply passage and the drain passage both communicating with the fuel supply and return chamber. The fuel supply passage is connected to a fuel tank via an external fuel pump, and the drain passage is connected to the fuel tank. The electromagnetic valve mechanism has a guide hole formed in the body, and a poppet-type valve member guided in the guide hole. The guide hole communicates at one end with the fuel supply and return chamber, and a valve seat is formed on the one end surface of the guide hole. The valve member includes a stem portion, and a head formed at one end of the stem portion. This stem portion has a reduced diameter portion disposed adjacent to the head. The stem portion is slidably received in the guide hole, and an annular space is formed between the reduced diameter portion and the inner peripheral surface of the guide hole. This annular space is in communication with a pump chamber. The electromagnetic valve mechanism also includes a stop portion facing the fuel supply and return chamber in opposed relation to the valve seat. The head of the valve member is disposed within the fuel supply and return chamber, and is movable between the stop portion and the valve seat. The electromagnetic valve further comprises an armature connected to the other end of the stem portion of the valve member, and a solenoid drive means for driving the armature. The solenoid drive means comprises a solenoid for urging the armature in such a manner that the valve member can be moved toward the valve seat, and a spring for urging the valve member in a direction away from the valve seat. The armature is received within an armature chamber formed in either the body or a casing fixedly mounted on the body.
In the above conventional unit fuel injector, during the suction stroke of the plunger, the solenoid is in its de-energized condition, so that the valve member is spaced apart from the valve seat. In this condition, the fuel is supplied from the external fuel pump to the pump chamber via the fuel supply passage, the fuel supply and return chamber and the annular space. When the solenoid is energized during the pump stroke of the plunger, the valve member is brought into engagement with the valve seat, so that the communication of the fuel supply and return chamber with the pump chamber is interrupted. As a result, the fuel within the pump chamber is pressurized and is injected from an injection nozzle mechanism. At this time, the fuel pressure is applied to the opposed shoulders formed respectively on the opposite ends of the reduced diameter portion of the valve member, so that the forces acting respectively on these opposed shoulders cancel each other. Therefore, the force required to be produced by the solenoid so as to hold the valve member in its closed position can be relatively small. When the solenoid is deenergized during the pump stroke of the plunger, the valve member is brought out of engagement with the valve seat under the influence of the spring, so that the fuel of high pressure within the pump chamber is spilled to the fuel supply and return chamber. As a result, the pressure within the pump chamber decreases, thus terminating the fuel injection operation.
At the moment when the valve member is disengaged from the valve seat so as to terminate the fuel injection operation as described above, the pressure within the fuel supply and return chamber becomes high, and the valve member is urged by this high pressure toward the valve seat. As a result, the speed of disengagement of the valve member from the valve seat under the influence of the spring may become slower, or the valve member may be instantaneously moved back toward the valve seat. This results in a problem that the area of flow between the valve seat and the valve member can not be increased quickly, so that the pressure drop in the pump chamber is retarded. As a result, the fuel injection operation fails to be completely terminated at a time, and the problem of subsequent dripping of the fuel is encountered.
In order to overcome this problem, the above-mentioned U. S. patents have proposed the following procedure. Specifically, the fuel supply and return chamber is communicated with the armature chamber by a passage, formed in the valve member, and/or a passage formed in the body. With this arrangement, the pressure of the above-mentioned spill fuel is applied not only to the fuel supply and return chamber but also to the armature chamber, so that substantially the same pressure acts on both of the opposite ends of the valve member, thereby canceling the forces acting on the valve member in opposite axial directions of the valve member.
With the electromagnetic valve mechanism of the above-mentioned U.S. patents, however, the fuel is filled in the armature chamber, and therefore the speed of movement of the armature is limited by the flow resistance of the fuel. As a result, the speed of movement of the valve member in response to the energization and de-energization of the solenoid becomes slow. Therefore, the speed of movement of the valve member in its opening direction becomes slow, which results in a risk that the valve member may fail to terminate the fuel injection operation at a time. Since the speed of movement of the valve member in its closing direction also becomes slow, the pressure within the pump chamber can not be increased at a proper speed.
In the fuel injectors of the above-mentioned U.S. patents, since the fuel supply and return chamber communicates with the fuel tank via the drain passage, there is a possibility that the above-mentioned spill fuel may escape to the drain passage. However, because of the provision of orifices in a drain pipe connected between the drain passage and the tank, such spill fuel pressure relief is quite insufficient. Incidentally, these orifices are required for the following reason. During the time when the fuel is supplied from the external fuel pump to the pump chamber via the fuel supply and return chamber, with the valve member disengaged from the valve seat, the escape of the fuel pressure from the fuel supply and return chamber is prevented because of the orifices, formed in the drain pipe, thereby maintaining the fuel supply pressure to the pump chamber at a predetermined level. In this connection, see column 7, lines 29 to 43 of the above-mentioned U.S. Pat. No. 4,392,612.
The above-described problems are also encountered in fuel injectors disclosed in U.S. Pat. Nos. 4,463,900 and 4,618,095 and similar in construction to the fuel injectors of the above-mentioned U.S. patents.
Japanese Laid-Open Utility Model Application No. 73570/88 discloses a unit fuel injector comprising an electromagnetic valve mechanism. The electromagnetic valve mechanism includes an armature chamber which at a glance, seems not to be filled with fuel. However, in this prior art publication, there is no clear description of a mechanism for relieving a spill fuel pressure.